1. Field of the Invention
The present invention relates generally to a radio communication system, and in particular, to an apparatus using transmitter antenna diversity to combat fading-caused degradation.
2. Description of the Related Art
An effective technique of relieving multi-path fading in a radio communication system is time and frequency diversity. Vahid Tarokh et al. has proposed a space-time block code. Space-time block codes are a well-known antenna diversity scheme disclosed in “Space Time Block Coding from Orthogonal Design”, IEEE Trans. On Info., Theory, Vol. 45, pp. 1456-1467, July 1999. Tarokh's space-time block code is an extension of transmitter antenna diversity as introduced by S. M. Alamouti, which allows use of two or more antennas. Tarokh's space-time block code is disclosed in “Space Time Block Coding from Orthogonal Design”, IEEE Trans. On Info., Theory, Vol. 45, pp. 1456-1467, July 1999, and Alamouti's space-time block code is found in “A Simple Transmitter Diversity Scheme for Wireless Communications”, IEEE Journal on Selected Area in Communications, Vol. 16, pp. 1451-1458, October 1998.
FIG. 1 is a block diagram of a conventional transmitter using a space-time block code as proposed by Tarokh. Referring to FIG. 1, the transmitter includes a serial-to-parallel (S/P) converter 110 and an encoder 120. It is assumed here that the transmitter uses three transmitter antennas 130, 132 and 134.
In operation, the S/P converter 110 provides the four symbols in each block to the encoder 120. The encoder 120 produces eight symbol sets from the four symbols and feeds the symbol sets to the three transmitter antennas 130, 132 and 134 over eight time intervals. The eight symbol sets can be expressed as shown in matrix (1):
                              g          3                =                  [                                                                                                       s                    1                                                                                        s                    2                                                                                        s                    3                                                                                                                    -                                          s                      1                                                                                                            s                    1                                                                                        -                                          s                      4                                                                                                                                        -                                          s                      3                                                                                                            s                    4                                                                                        s                    1                                                                                                                    -                                          s                      4                                                                                                            -                                          s                      3                                                                                                            s                    2                                                                                                                    s                    1                    *                                                                                        s                    2                    *                                                                                        s                    3                    *                                                                                                                    -                                          s                      2                      *                                                                                                            s                    1                    *                                                                                        -                                          s                      4                      *                                                                                                                                        -                                          s                      3                      *                                                                                                            s                    4                    *                                                                                        s                    1                    *                                                                                                                    s                    4                    *                                                                                        s                    3                    *                                                                                        s                    2                    *                                                                        ]                                              (        1        )            where g3 is an 8×3 coding matrix of symbols transmitted through the three transmitter antennas and s1, s2, s3, s4 are four input symbols to be transmitted.
As shown above, the encoder 120 outputs eight symbol sets produced from the four input symbols and their negatives and conjugates to the three transmitter antennas 130, 132 and 134 for eight time intervals. Notably, symbol sequences for the transmitter antennas, that is, rows, are orthogonal to each other.
More specifically, the first three symbols s1, s2, s3 are provided to the three antennas 130, 132 and 134, respectively for a first time interval. In the same manner, the last three symbols s4*, s3*, s2* are provided to the antennas 130, 132 and 134, respectively for the last time interval. That is, the encoder 120 sequentially provides symbols in an mth column of the coding matrix to an mth antenna.
FIG. 2 is a block diagram of a receiver for receiving signals from the transmitter illustrated in FIG. 1. Referring to FIG. 2, the receiver includes a plurality of receiver antennas 140 to 142, a channel estimator 150, a multi-channel symbol arranger 160, and a detector 170.
In operation, the channel estimator 150 estimates channel coefficients representing channel gains from the transmitter antennas to the receiver antennas. The multi-channel symbol arranger 160 collects symbols from the receiver antennas 140 to 142. The detector 170 then obtains hypotheses by multiplying the received symbols by the channel coefficients, calculates a decision statistic for all possible symbols using the hypotheses, and detects transmission symbols by threshold detection.
Alamouti's space-time coding achieves a maximum diversity order, that is, a diversity order identical to the number of transmitter antennas without data rate loss despite transmission of complex symbols through two transmitter antennas. This is expanded by Tarokh's space-time coding implemented in the transmitter and receiver illustrated in FIGS. 1 and 2. According to this scheme, a maximum diversity order is achieved using space-time block codes in the form of a matrix having orthogonal rows. The transmission of four complex symbols for eight time intervals, however, leads to decreasing the data rate by half. Since eight time intervals are taken to transmit one whole block (i.e. four symbols), reception performance is degraded due to the change of channel environment for the block in the case of fast fading.
As described above, when transmitting complex symbols through three or more antennas using conventional space-time block coding, 2N time intervals are required to transmit N symbols, resulting in a data rate loss. Therefore, transmission latency is lengthened and data rate is lowered.